1. Field of the Invention
This invention relates to a packaged piezoelectric oscillator of the type which itself incorporates a pair of capacitors. The present invention also relates to a method of making a plurality of such oscillators at a time.
2. Description of the Related Art:
A packaged piezoelectric oscillator incorporating a pair of capacitors is conventional. For better understanding, a typical example of such an oscillator is illustrated in FIGS. 45 and 46 of the accompanying drawings.
As shown in FIGS. 45 and 46, specifically, the prior art packaged piezoelectric oscillator 4 comprises a piezoelectric element 1 enclosed in an insulating package which consists of an elongate base plate 5 and a cover member CM. The piezoelectric element 1 includes a ceramic bar 1a, a lower electrode film 1b formed on the lower surface of the ceramic bar 1a, and an upper electrode film 1c formed on the upper surface of the ceramic bar 1a. The respective electrode films 1b, 1c may be made of silver by sputtering for example. The respective electrode films overlap each other longitudinally of the ceramic bar 1a over a predetermined area, and this overlapping area plays an important role in determining the characteristics of the piezoelectric element 1.
The base plate 5 is made of a dielectric substance. The base plate 5 is formed with first to third lead electrodes 6, 7, 8 spaced from each other longitudinally of the base plate. The first and second lead electrodes 6, 7 are held in electrical conduction with the respective electrode films 1b, 1c of the piezoelectric element 1 by deposits 9 of electrically conductive adhesive, so that the first and second lead electrodes 6, 7 are utilized to supply electric power for piezoelectric oscillation. Further, due to the dielectric nature of the base plate 5, two capacitors 3 are provided respectively between the first and third lead electrodes 6, 8, and between the second and third lead electrodes 7, 8.
For mounting on a circuit board (not shown), the above-described piezoelectric oscillator 4 is generally arranged in parallel to a CMOS inverter 2 (CMOS: complementary metal oxide semiconductor), as shown in FIG. 27. Further, the first and second lead electrodes 6, 7 of the piezoelectric element 1 are grounded through the respective capacitors 3 which work for phase inversion.
According to the prior art described above, since the piezoelectric oscillator 4 itself incorporates the capacitors 3, it is possible to realize a compact circuit arrangement in comparison with separately providing capacitors. Further, the prior art packaged piezoelectric oscillator 4 is also advantageous in that the user need not care about the capacitance of the capacitors 3 which is selected by the supplier.
However, the prior art piezoelectric oscillator is still disadvantageous in the following respects.
First, since the respective lead electrodes 6, 7, 8 for the capacitors 3 are directly exposed to the circuit board to which the piezoelectric oscillator 4 is mounted, the dielectric constant of the base plate 5 may be likely to vary under the influences of nearby conductor parts on the circuit board such as wirings and/or solder deposits. As a result, the capacitance of the capacitors may vary to result in fluctuation of the oscillation characteristics of the piezoelectric oscillator 4.
Secondly, since the base plate 5 works dually as a capacitor body and as a member for forming the respective lead electrodes 6, 7, 8, there is a limitation in optionally selecting the capacitance of the respective capacitors 3.